1. Field of the Invention
The present invention relates generally to presses and the like having a ram and upper and lower tools for processing workpieces such as sheet metals and, more particularly, to a method and apparatus for providing a safety device against troubles or malfunctions of the tools of the presses.
2. Description of the Prior Art
As is well known, presses are provided with a vertically movable ram and upper and lower tools or dies which are worked by the ram to cooperate with each other to process workpieces such as sheet metals. In most cases, the upper and lower tools are unitized as a tool assembly or die set to be mounted on a work-table just beneath the ram for convenience of installation and for other reasons. Otherwise, the upper and lower tools are held on a tool holding means, such as a pair of turret members, which are so designed as to hold a number of upper and lower tools and selectively bring a desired pair of upper and lower tools into position just under the ram.
In the case where the upper and lower tools are employed as an unitized tool assembly or die set or used on holding means such as turret members, the upper tool is so arranged as to be drawn up or stripped by a stripping spring out of a workpiece to be processed after each completion of processing cycles. More particularly, such a stripping spring is so disposed as to be compressed when the ram is urging the upper tool toward the workpiece and the lower tool and then lift up or strip the upper tool out of contact with the workpiece.
Especially in punching and blanking operations, however, the upper tool will often fail to be stripped out of the workpiece after a completion of a processing cycle. This arises from various causes, such as breakage or fatigue of the stripping spring and wear or thermal expansion of the upper tool. Of course, when the upper tool is mis-stripped or not stripped out of the workpiece in punching and blanking operations, it often happens that the upper tool will be caught not only in the workpiece but also in the lower tool. Anyway, it is very dangerous when the upper tool is mis-stripped or fails to be stripped from the workpiece, since the press will go on moving with the upper tool caught in the workpiece. Also, any or all of the upper and lower tools, the workpiece and the press will be damaged or broken if the workpiece is forcibly moved by power when the upper tool is caught in the workpiece because of mis-stripping. Since workpieces are usually moved or fed into presses automatically by power, especially in punching and blanking operations, it has been disadvantageous that damages to tools, workpieces and presses frequently occurs when the upper tool is caught in the workpiece because of mis-stripping.
For the above described reasons, it is necessary to move and feed the workpiece after the upper tool has been completely stripped from the workpiece without being mis-stripped. Of course, it is necessary to stop the workpiece from being moved and to also stop the press from being driven the moment the upper tool is caught in the workpiece because of mis-stripping, especially when the workpiece is being automatically fed by power. In other words, it is necessary to detect mis-stripping of the upper tool to stop the workpiece and the press from being moved the moment the upper tool is mis-stipped.
Heretofore, various attempts have been made to detect mis-stripping of upper tools in presses in order to stop workpieces and presses from being moved the moment mis-strippings occur. For example, a photoelectric tube is employed so that it may check each return of the upper tool to its normal position after each completion of processing cycles so as to stop the workpiece and the press when the upper tool is not normally returned to its position. Generally, the conventional arrangement has been such that each stripping of the upper tool from the workpiece is checked when the upper tool has passed or cleared, in returning, a fixed point at which it is to be stripped out of a workpiece of the maximum thickness which can be processed. Therefore, in the conventional devices for detecting mis-stripping of the upper tool, the workpiece cannot be moved until the upper tool has passed or cleared a fixed point whether workpieces being actually processed are large or small in thickness. Accordingly, it has been disadvantageous with regard to conventional mis-stripping detecting devices that the processing speed is limited in spite of the fact that a millisecond matters in punching and blanking operations. Also, it has been disadvantageous that the conventional mis-stripping devices are apt to perform erroneously from various causes when no mis-stripping occurs.